There was a time when the use of breast sonography was limited to determining if a mass initially detected with mammography or by palpation was a "cyst vs. solid lesion," but those days are long gone. Dramatic improvements in ultrasound imaging (US) technology have elevated the modality to its current status as an essential tool for assessment of patients with suspected breast abnormalities and to guide interventional procedures.
The use of US technology has also been recognized as an important addition to screening mammography, particularly for patients with dense breasts. The efforts of special interest groups have led to both greater patient awareness and recommendations-and in some cases requirements-that US be included for breast cancer screening examinations. These and other advances promise to keep sonography at the forefront in the battle against breast disease.
Breast cancer screening
First recommended for routine screening by the American Cancer Society in 1980, mammography remains the only modality approved for breast cancer screening. Although digital mammography and other advances have considerably improved the modality's diagnostic capabilities, mammography still has several well-recognized limitations. In 2010, the Society of Breast Imaging (SBI) and the American College of Radiology (ACR) published recommendations regarding breast cancer screening with a variety of imaging modalities.
The recommendations suggest that screening MRI be used as an adjunct to mammography for patients who are at high risk for breast cancer. Several studies have confirmed that MRI can detect cancers that were missed on mammography, and published reports indicate that the 71 to 91 percent sensitivity of MRI for detecting breast cancer is considerably better than that of mammography. However, MRI is expensive, requires contrast injections and many patients choose not to have the procedure. Furthermore, MRI is not widely available in some locations and the SBI/ACR recommendations indicate that screening MRI is not appropriate for women who have a less than a 15 percent lifetime breast cancer risk. An alternative to MRI is needed to complement screening mammography, and numerous published reports suggest that sonography has that potential.
Studies have demonstrated that US can identify mammographically occult malignant tumors that that are at an earlier stage, potentially leading to more effective therapy and patient outcomes. One of the most widely cited clinical trials is the ACR Imaging Network trial (ACRIN 6666). This well-controlled, prospective, multicenter trial included 2,637 patients who were considered at increased risk for breast cancer. Patients received mammography and US studies, and the two exams were read independently. A total of 41 cancers were diagnosed in 40 patients. The diagnostic accuracy of mammography was 78 percent, which improved to 91 percent when the two modalities were combined. The addition of US to mammography increased the breast cancer detection rate 4.2 per 1,000 women studied (which may not seem like much unless you or a loved one were one of the 4.2 women).
Sonography is particularly beneficial for evaluation of patients who have dense breasts. This is an important attribute of the modality because more than 70 percent of breast cancers occur in women who have dense breasts. Although radiologists know that dense breasts pose additional risks for the patient as well as challenges for mammography, only recently have patients become aware of these issues. Are You Dense? (www.areyoudense.org) is a public charity that ".is dedicated to informing the public about dense breast tissue and its significance for the early detection of breast cancer." Nancy M. Cappello, PhD, a breast cancer survivor, is president of AYD. She campaigned for greater awareness of the dangers of dense breasts and her efforts eventually lead to the first state law (enacted on October 1, 2005) that requires insurance companies in Connecticut to pay for breast sonograms when recommended by a physician. Connecticut laws also require physicians to inform patients, in writing, that they have dense breasts.
Several other states are considering similar breast density laws. At the federal level, The Breast Density and Mammography Reporting Act of 2011 was introduced at the end of last year. Like the Connecticut law, the federal act is intended to help raise awareness of breast cancer risks and the importance of appropriate screening by requiring that patients be informed about their breast density.
Although there are many benefits of US as a modality (e.g., availability, low cost, lack of ionizing radiation, etc.), there are several problems in terms of using it for bilateral whole-breast diagnostic examinations-not to mention routine breast cancer screening. Breast US requires considerable time and expertise and there is the potential for both false-negative as well as false-positive findings. Furthermore, comparing serial US studies poses challenges for the interpreting physician and there have not been any studies that demonstrate that whole-breast US reduces patient mortality.
One potential solution for some problems of performing breast sonography (for both diagnostic exams as well as screening) could be greater use of automated scanners. There are several commercially available systems in the U.S. including the Acuson S2000 ABVS (Siemens Medical Solutions, Malvern, Pa.) and the somo•v™ Automated Breast Ultrasound System (ABUS; U-Systems Inc., Sunnyvale, Calif.).
Automated scanners obtain volume US data that is then reviewed on a workstation. Because data acquisition is typically more rapid than conventional scanning techniques (i.e., using handheld transducers) and is automated, it reduces the operator dependency of exam performance. The acquired images can be viewed in conventional scanning planes as well as in planes that cannot be obtained using conventional scanning methods, most notably the coronal plane; this improves lesion conspicuity and facilitates comparisons to mammography as well as to subsequent US examinations, thereby enhancing interpretation.
The somo•v ABUS system can acquire volume US scans of both breasts in about six minutes and methodically captures up to 350 images. A U.S. multicenter clinical trial sponsored by U-Systems Inc., SOMO•INSIGHT, seeks to determine if ABUS screening studies combined with a routine screening mammography is more accurate in detecting breast cancer in women with dense breast tissue than having screening mammograms alone. Large-scale clinical trials such as this are necessary to determine the ultimate potential of automated US for breast screening procedures.
An alternative to palpation
Palpation has been an exam mainstay for centuries. The goal of palpation of the breast is to identify areas that feel different (i.e., softer or harder) than the surrounding breast tissue. Recent U.S. Preventive Services Task Force guidelines state that clinical breast examinations (CBEs) performed by a physician have a sensitivity of 40 to 69 percent and a specificity of 88 to 99 percent, while a patient breast self-examination (BSE) has a sensitivity of just 12 to 41 percent. Thus, the USPSTF recommends against teaching patients how to perform BSEs. Although CBEs are still widely practiced and have clinical value--especially where mammography access is limited--the CBE alone is not considered adequate for effective screening.
Advances in US elastography have resulted in the ability to replace the physician's fingers with an imaging procedure to provide a means to assess tissue stiffness in a more objective and reproducible manner. Systems that use either manual tissue compression or shear wave elastography are now commercially available. The latest advances in elastography, or E-mode, include the ability to quantify tissue stiffness.
A recently published study by Barr, et. al, assessed compression elastography for characterizing breast lesions as benign or malignant. More than 630 breast masses were evaluated with the imaging results compared to US-guided biopsies. The investigators compared measurements of the breast masses on a conventional B-mode US image and on the elastogram and calculated an E-mode/B-mode ratio. Masses with a ratio >1.0 were considered malignant. There were 222 malignant lesions and 413 benign lesions as determined by pathology. Elastography had an overall sensitivity of 99 percent and overall specificity of 87 percent, suggesting that elastography combined with conventional US can improve the ability to characterize breast tumors.
Studies in contrast
There have been several published reports on the use of contrast-enhanced ultrasound imaging (CEUS) for breast applications. The technology is used to characterize breast masses and also to monitor breast cancer therapies. Reports show mixed results regarding the value of CEUS for breast lesion characterization, so it's not considered a viable alternative to breast lesion biopsies. Research is ongoing.
Patients diagnosed with breast cancer receive surgery to remove the lymph nodes that receive drainage from the tumor (i.e., sentinel lymph nodes, SLNs) to stage and manage their disease. The most common techniques used for lymphatic mapping is injection of radiotracers (i.e., pre-operative lymphoscintigraphy or intraoperatively with a gamma probe) or by injection of blue dye and visual identification of dye-stained nodes during surgery. However, both of these established methods have limitations or contraindications. Peritumoral injection of an ultrasound contrast agent followed by CEUS to identify contrast-enhanced SLNs has been investigated as an alternative lymphatic mapping technique, with the results of investigations in animals and preliminary clinical studies encouraging.
The recognized safety, low-cost and wide availability of US technology makes it an ideal imaging modality for breast imaging applications. The diagnostic capabilities of sonography are well recognized and the use of US as a guidance tool for interventional procedures is growing. Advances in technology will continue and enhance the use of sonography for diagnostic examinations, screening studies and interventional procedures.
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Daniel A. Merton is the technical coordinator of research at the Jefferson Ultrasound Research and Education Institute, Department of Radiology, Thomas Jefferson University Hospital in Philadelphia.